Deciphering buried air phases on natural and bioinspired superhydrophobic surfaces using synchrotron radiation-based X-ray phase-contrast imaging

Superhydrophobicity is an important phenomenon in nature that inspires the design of numerous biomimetic functional materials. A superhydrophobic surface is expected to have a three-phase solid–liquid-vapor interface. To directly image the buried air phase in wetted superhydrophobic surfaces, we employed synchrotron radiation-based X-ray phase-contrast imaging to non-invasively probe the surfaces of natural lotus leaves and artificial carbon nanotube films in three dimensions. Reconstructed images of the three-phase distribution surrounding the superhydrophobic surfaces were presented with high resolution and contrast. Further extraction of the phases enabled direct analysis of the relationship between the surface morphology and its wetting property. We found that the protruding micro/nano-structures trapped a layer of air up to 14 μm thick, which played an important role in the observed superhydrophobicity. Direct evidence of the presence of a buffer layer air cushion deepens our understanding of hydrophobicity and opens new opportunities for designing novel bioinspired materials. Synchrotron X-ray imaging has given researchers the clearest view yet of how trapped air pockets can make surfaces extremely water repellent. Electron microscopy has helped researchers determine that rough, micro and nanoscale textures on natural surfaces such as lotus leaves cause water droplets to bead up. But below the droplets are hidden air cushions that elude direct observation. Now, Chunhai Fan and colleagues in Shanghai, China, have used non-invasive synchrotron radiation to clarify the role of the air phase in superhydrophobic lotus leaves and carbon nanotube films. High-colour and high-resolution images of interfacial electron density revealed that individual air pockets prevent water from infiltrating the surfaces grooves and can connect into large cushions that dramatically reduce the solid-water contact area. Any protruding structures on the superhydrophobic surfaces serve mainly as droplet anchor points. Synchrotron radiation-based X-ray phase-contrast imaging was employed to observe the solid–liquid-vapor interface on superhydrophobic surfaces. The reconstructed 3D images revealed that the water exists in the Cassie-to-Wenzel transit state. The micro/nano-structures trapped a large amount air on the supherhydrophobic surface to repel water.